As well known, the difficulty of counterfeiting can be increased by adding security threads, or special security inserts and labels of various shapes inside or on the surfaces of security papers or plastic polymers such as banknotes, securities or bill notes. These materials can be added in the security papers or plastic polymers in the forms of polyester threads/tapes or strips with special shapes during the process of fabrication, and can also be attached to the surfaces of the security papers or plastic polymers at the final process of fabrication. These security materials are usually cut or processed from security thin films to form security threads, security tapes, and security inserts or labels of various shapes.
As an effective measure against counterfeit, security threads have long been used for security papers such as banknotes. Earlier security threads were applied as metal threads having circular cross-sections that were positioned longitudinally along the paper machine and continuously applied. A later developed security thread typically was a metallized polymer, whose carrier was flat-shaped in cross-section. Such a flat-shaped security thread is applied in two modes in security papers, one of which is the open mode, wherein the security thread is partially embedded inside the paper and partially exposed on the paper surface; another is the insertion mode, wherein the entire security thread is completely embedded in the paper. Embedment of the security thread into the security paper provides a primary public security visible to the naked eye.
To enhance anti-counterfeit performance of the security thread, people has provided it with optical security information and/or magnetic security information during the process of fabricating the security thread. Chinese patent application CN 1715560A discloses a security thread for enhancing security of paper. This security thread contains both optical security information and magnetic security information, as the security thread has a holographic security layer formed of a molded layer and a metal reflection layer with optical security images, and has a magnetic security information layer formed of magnetic particles and resins such as cellulose. During the process of applying the security thread to the security paper, the security thread tends to be longitudinally elongated due to longitudinal traction of the paper machine, and when the security thread is longitudinally elongated to a certain extent, a portion of machine-readable magnetic security information can be lost, which causes inferior stability of machine-readable electromagnetic characteristics to the detriment of anti-counterfeit performance.
On the other hand, the conventional magnetic security thread is fabricated by coating a magnetic material having a purely metallic type or metallic ferromagnetic powders or coating printing ink on substrates such as PET. Its disadvantages are the necessity to use nonmagnetic substances such as adhesives and the requirement for the thickness to reach about several microns, whether it is coating a magnetic material or printing magnetic security ink. Consequently, the fabricated security thread appears too thick relative to the material of a thin film type. Placement of such a magnetic security thread in a sheet of paper would render it difficult to smooth the surface of the paper, thus affecting subsequent printing process. Moreover, the thickness of several microns of a printed magnetic security functional ink layer can also create environmental problems due to volatilization of organic solvents in the preparation process.
The security tape is usually adhered on the surface of a security paper or a security plastic polymer, and the security tape may contain many types of security information. U.S. Pat. No. 4,684,795 (Security Tape with Integrated Hologram and Magnetic Strip) discloses a security tape for use on identification cards, wherein a magnetic layer containing a ferrous oxide as the main component is sandwiched between an optical security nonmagnetic layer with a hologram and a plastic base layer of the identification card. The magnetic layer in this patent still belongs to conventional magnetic security, i.e., it is still a security measure that makes use of a magnetically hard material to provide machine-readable signals.
Amorphous alloy materials have many excellent characteristics, and have been a focus of attention in research, development and application in the materials science as a novel functional material. Relatively common applications include using these materials as composite reinforcing materials in strengthening component parts of products; there are also many applications as soft magnetic materials for use in various transformers and sensors. Applications of amorphous materials in the field of security detection are limited to antitheft labels sandwiched in books or goods in the form of amorphous alloy strips, wherein alarm is triggered upon receiving a change in waveforms by the device. For instance, international application WO 00/05693 relates to an antitheft label made of multilayered film of 100-2000 nm containing amorphous soft magnetic thin film. Despite the described alloy component being suitable for use as antitheft label, one defect of the technical solution of this invention lies in its over simplification for use in the field of advanced anti-counterfeit of securities, exhibited as no definite directional requirement of the detection signal. Moreover, for use as an antitheft label, there are also certain requirements on the amorphous film as demanded by the detection sensitivity: first, the film should have certain thickness (to enable three-dimensional detection, it is usually required that the thickness of the thin film is about 1000 nm for use in the antitheft label); and to facilitate detection, it is also desirable to minimize the interference originated by the material itself; therefore, there are also specific requirements on the elemental composition of the amorphous film.
It is because the property of the amorphous alloy is closely related to its composition, although currently there have been many reports on amorphous alloy materials and their applications, so far there is no report about research on utilizing the amorphous alloy materials to provide the security information layer.